1. Field of the Invention
The present invention relates generally to electronic digital cameras, and more particularly to a digital camera using a pre-flash in combination with digital camera image acquisition apparatus and a processor for creating a histogram to determine an optimum flash power controlled through calculation of flash capacitor voltage.
2. Brief Description of the Prior Art
Prior art cameras have used many different techniques to achieve optimum exposure, from hand held light meters to built in automatic exposure systems with flash. One method of controlling exposure is based on "through the lens" flash control in which the flash is terminated when sufficient light is collected by a photo receptor in the camera. Another method uses an infrared photo diode to measure the light. The advantage of using infrared is that in the infrared zone, light is more evenly reflective as a function of color in the visible spectrum. These methods are all based on an average (integration) over the entire image, and are not able to separate out important image areas for priority in setting the amount of light for exposure. For example, a combination of a close image and a distant background will result in a "washed out" foreground. Since primary subjects are often in the foreground, this is a serious problem in automatic exposure systems. These exposure control systems also require a very fast electronic switching device for a fast flash and a separate photo receptor which add complexity and cost to the system. The infrared receptor also has a problem in that the light measured is only a monochromatic estimation of the scenery. This estimation may be close in some cases, but in others, it accentuates the problem of film/CCD metamerism, a condition where different wavelengths in a scene are improperly recorded.
In Coltman et al., U.S. Pat. No. 4,998,128 the reflectivity of a subject is determined by pulsing a flash unit for predetermined short period of time. The light is detected by both visible and infrared light detectors, the outputs of which are integrated and used to select an optimum aperture and speed setting for taking the picture. In Taylor, U.S. Pat. No. 4,066,884 an adjustable filter is used to vary the light intensity from an electronic flash unit, the degree of adjustment being empirically determined for a particular type of camera, in this case cameras designed for use with explosive flashbulbs which have relatively long duration of light intensity. The problem with the electronic flash unit when used with cameras having automatic exposure adjustment is that the time duration of the electronic flash is too short for the automatic exposure system to work. In Winter, U.S. Pat. No. 4,549,801, an electrically operated flash camera employs an infrared flash reflected light signal stored in a single memory storage to control focus and aperture. In Ishida, U.S. Pat. No. 4,256,995, an electronic flash is disclosed which is capable of emitting light for a longer duration of time so as to allow automatic exposure control camera systems to function. Kabayashi et al., U.S. Pat. No. 4,717,934 discloses a flash used prior to image acquisition to determine the amount of flash required for an adequate exposure. This is done by detecting and integrating light radiated directly from the flash, and integrating the reflected light from the object. The flash power is provided by a separate capacitor from the capacitor used for the main flash.
In Coltman, the pre-flash system functions independently from the camera image acquisition apparatus, depending on a predetermined look up table. The accuracy of this method is limited to the exactness of the correlation between the look up table and the actual setting. The mechanical adjustment device of Taylor for control of the flash intensity is dependent on the skill of the operator in knowing where to set the flash cover. In Winter, the burden of adjusting for exposure is placed entirely on the camera aperture and shutter speed. No attempt is made to control the amount of flash. The device of Ishida similarly does not use control of the flash time as an aid in achieving proper exposure but simply provides a flash of long duration, allowing conventional camera automatic exposure systems to function as if the lighting were ambient. This system would consume larger amounts of flash power than what would otherwise be required for proper exposure. The device of Kabayashi requires a separate capacitor for pre-flashing, which involves extra cost and space.
It is apparent from the above references that an improved camera is desirable, that conserves flash power and minimizes cost and space. Also, an improved camera would provide a method for evaluating light from different parts of an image to determine optimum exposure of particularly selected areas, this being a particular problem when objects are at various distances from the camera and when they are in contrast to each other. Such a camera would be a significant improvement over the prior art.